1873.] DKEW UPPER-INDUS BASIN. 447 



level of that valley. The radii of the fan are about a mile long ; 

 the slope of the ground along these radii (which are each in the di- 

 rection of the greatest slope) is five or six degrees. The fan is 

 properly a flat cone, having its apex at the mouth of the ravine. In 

 this instance the length of the axis, that is to say, the vertical height 

 of the apex above the alluvial plain, will be about 500 feet, the length 

 of the base of the generating triangle being about a mile. It will 

 be observed how very straight is the line of the profile. This is 

 highly characteristic of these fans ; and the character is equally 

 marked whatever portion of it we get into view, whatever radius 

 comes into profile. The hard, straight bine among the irregular out- 

 lines of the mountains adds a strange and unlooked-for feature to 

 the landscape. The radial lines seen in fig. 3 are as faithful repre- 

 sentations as I could make of the watercourses with which the sur- 

 face of the fan is scored ; whether we start from the furthest project- 

 ing point of the circumference, or edge along the mountains 

 (against which the fan abuts, ending off sharply) to go to the 

 apex at the ravine's mouth, we are always on an equal slope, in this 

 case of 5° or 6°, as before said. In walking across one of these 

 large fans along the path, which is usually made in a curve some- 

 where between the arc and the chord, one is apt to be continually 

 expecting in a few steps to arrive at the summit of the slope ; but 

 again and again is one disappointed, new portions of the cone inter- 

 vening in succession, until the central radius is reached. 



The mode of formation of this fan it is not difficult to trace. 

 Granting the stream of the side-ravine to be carrying down such an 

 amount of detritus as to cause it to be an accumulating, rather 

 than a denuding, stream, and there being such a relation between the 

 carrying-power of the water and the size of the material as to allow 

 of this remaining at a marked slope, we have before us all the con- 

 ditions necessary. When the alluvial matter which had been ac- 

 cumulating in the ravine reached past its mouth, there was a 

 tendency of the stream to flow over the material it was bringing 

 down, now in one direction, now in another — in every direction, 

 indeed, from the mouth of the gorge as a centre ; and along each 

 line, as it flowed, it accumulated material at an equal angle : thus 

 cone after cone was formed, each coating the last, and the sloping- 

 fan both rose and spread. Coincident with this there must, in most 

 cases, have been a rising of the bed of the stream back within the 

 ravine. The regularity of the cone was preserved by this cause — 

 namely, that if at any time there was an increase only in one part, 

 say in the direction straight out from the mouth, it could be but 

 temporary ; for the next tendency of the water would be to flow, not 

 along that raised part, but off on one side of it, where, still accumu- 

 lating, it would raise the level of another portion ; and so, all the 

 lowest parts being reached by the detritus-bearing water, none but 

 very small uncvcnncsses could occur. 



It follows from these considerations that the material accumulates 

 in a general way in layers, but in layers of a peculiar form, not hori- 

 zontal, rather curved coatings. And indeed this can often be observed 



2 g2 



